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>Posted on 19 Jun 1995 at 14:17:19 by Jim Greenlee

>> When I first heard of blending nitrox using the partial pressure
>> method, I was told that this leads to stratification in the tank. My
>> experience in chemistry tells me that this is impossible,
>
>Well, I was a Doubter, too, until I actually *saw* it happen. The
>particular incident occurred with a tank that I mixed *myself* (I am
>an IANTD Nitrox Instructor and Gas Blender). The tank was a Genesis
>80, which started out with a couple hundred PSI of a standard mix (32
>or 36, can't remember which).

>At any rate, I did the math and added however much pure O2 I needed
>for the mix I wanted, topped with air to the tank's rated pressure
>(3500 PSI), and then checked the mix with a Mini-Ox. To my surprise,
>the FO2 read significantly lower than I expected, so I checked the
>FO2 with another analyzer - same result. I then re-did the math, but
>all the numbers checked out.
>
>The owner of the dive shop happened to be standing there while I was
>doing all this, and suggested that the mix might have stratified, and
>said I should roll the tank back and forth for a few minutes to see
>if the FO2 changed. I was skeptical, but decided to try it anyway,
>rather than blow a whole fill and have to start over.
>
>Well, I rolled the tank for a few minutes, re-checked with the
>Mini-Ox, and sure enough, the FO2 had gone up by a few percentage
>points. After a few more minutes of rolling, the FO2 had risen to
>what my "target" was, after which it leveled off (note that I did not
>adjust the calibration on the Mini-Ox at any point).
>
>I have only observed this phenomenon just the one time with that one
>tank, but since I could not come up with any other explanation for
>what I observed, then I assume it must have been the result of
>stratification.
>
>So now I am a Believer. Any Doubters out there care to shoot holes in
>what I saw?

I'd like to.

First, I'd say that rolling impossibly can have any measurable effect.
This should be easy to test.

Second, the tank cooled during this rolling and I would have expected
that the pO2 would go up as the tank was allowed to cool.

Thirdly, does the density of the gas that's added effect the
temperature rise in the tank being filled ?  If yes the very
method of filling matters.

Fourth: I'll resist the notion of stratification until someone
positively measures a *decrease* in pO2 with time (mixing).
Barring errors in the measurement gear this would once and for
all demonstrate that O2 pockets indeed did exist inside the tank.

john

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